This application relates to the discovery that high melting isotropic pitches can be converted to solvated isotropic pitches thereby lowering the effective melting point of the pitch. Solvated pitches prepared by the disclosed process may be spun into fibers which require little or no stabilization treatment.
The processes for spinning traditional non-solvated isotropic pitches are well known. Currently Kureha Chemical Industry Co. is the leading producer with a capacity of 900 tons/year. Additional manufacturers include Ashland Oil Co. and Kawasaki Steel Company.
Isotropic carbon fibers are commonly used as reinforcement for concrete structures. In this aspect, carbon fibers must compete with steel and fiberglass fibers. Therefore, it is desirable to provide carbon fibers at the lowest cost possible. In the process of manufacturing carbon fibers, one of the slowest and costliest steps is the stabilization (usually by oxidation) of the as-spun fiber prior to the carbonization of the fiber. The stabilization step is necessary to preclude melting of the fiber during the carbonization process which occurs at temperatures in excess of 350.degree. C. and frequently higher than 1000.degree. C. In order to reduce the time and cost of this step, one preferably manufactures fibers from high melting pitches. However, prior to the present invention, those pitches which melted above the spinning temperature were unusable.
Therefore, one of the objects of the present invention is to provide a process for manufacturing carbon fibers which do not require oxidation prior to stabilization. Additionally, the present invention provides a solvated isotropic pitch which has a fluid temperature at least 40.degree. C. lower than the melting point of the same pitch in the non-solvated state. Further, the present invention provides a solvated pitch which can be spun into a fiber, devolatized and oxidatively stabilized at a temperature equal to or greater than the spinning temperature.
Definitions
For the purposes of this specification and claims, the following terms and definitions apply:
"Pitch" as used herein means substances having the properties of pitches produced as by-products in various industrial production processes such as natural asphalt, petroleum pitches and heavy oil obtained as a by-product in a naphtha cracking industry and pitches of high carbon content obtained from coal. PA1 "Petroleum pitch" means the residual carbonaceous material obtained from the catalytic and thermal cracking of petroleum distillates or residues. PA1 "Petroleum coke" means the solid infusible residue resulting from high temperature thermal treatment of petroleum pitch. PA1 "Isotropic pitch" means pitch comprising molecules which are not aligned in optically ordered liquid crystal. PA1 "Anisotropic pitch" or "mesophase pitch" means pitch comprising molecules having aromatic structures which through interaction are associated together to form optically ordered liquid crystals, which are either liquid or solid depending on temperature. PA1 "Mesogens" means molecules which when melted or fused form mesophase pitch and comprise a broad mixture of large aromatic molecules which arrange upon heating to form liquid crystals. PA1 "Pseudomesogen" means materials which are potentially mesophase precursors, but which normally will not form optically ordered liquid crystals upon heating, but will directly form a solid coke upon heating, such that there is no melting or fusing visible. PA1 "Fluid temperature" for a solvated pitch is determined to be the temperature at which a viscosity of 6000 poise is registered upon cooling of the solvated pitch at 1.degree. C. per minute from a temperature in excess of its melting point. If the melting point of a solvated pitch could be easily determined, it would always be lower than the fluid temperature. PA1 "Solvated pitch" means a pitch which contains between 5 and 40 percent by weight of solvent in the pitch which has a fluid temperature of at least 40.degree. C. lower than the melting point of the pitch component when not associated with solvent. PA1 "Fibers" means lengths of fiber capable of formation into useful articles. PA1 "Oriented Molecular Structure" means the alignment of mesophase domains in formed carbon-containing artifacts, which alignment corresponds to the axis of the artifact and provides structural properties to the artifact. PA1 "Solvent Content" when referring to solvated pitch is that value determined by weight loss on vacuum separation of the solvent. In this determination, a sample free of entrained or trapped solvent is accurately weighed, crushed and heated in a vacuum oven at less than 5 mm pressure and at a temperature of 150.degree. C. for one hour. The percent solvent content is the weight loss or difference in weight times 100 divided by the original sample weight. PA1 "Oxidation/Stabilization" is the process of making a pitch artifact infusible or unmeltable by reacting the artifact with oxygen or an oxidizing agent. PA1 "Softening and Melting points" are determined by heating a sample at about 5.degree. C./minute on a hot stage microscope under an inert atmosphere. The softening point for a dried pitch is the first rounding of angular features of the pitch particles. The melting point for a dried pitch is that temperature at which the first observable flow of the softened pitch is seen.